From an invoice, I click Register Payment.
However, Payment Method includes only two items: Cash (USD) and Bank (USD).
My invoice, journal, and accounts relating to this transaction are all in NGN and that's how I want to record the payment.
Where can I add a new Payment Method such as Cash (NGN)?

2) That the TYPE of Journal is important, can be found by looking at the XML that defines the dropdown. You can activate developer mode (https://accounts.openerp.com/forum/Help-1/question/83/) and examine the XML that defines the form view. (The Edit FormView option in the debug dropdown on the Payment screen). You can see the XML definition for payment method is:

Thanks, works perfectly! Your first sentence was the key. To guide my self-education, is there somewhere I should be looking other than https://doc.openerp.com/contents/#contents-link to find this kind of thing?

In
order to create a Payment Method is POS in Odoo , the following steps
needs to be followed :-

First thing which is required is to be done is to create a ACCOUNT,
by following ACCOUNTING>CONFIGURATION>ACCOUNTS.
We can either copy any exiting account or create a new one depending
on the requirement

After creating an account, we need to create a JOURNAL, which
is in turn a PAYMENT METHOD for our Point Of Sales.
During this journal creation we would assign the account created by
us in the previous step. The Journal can be created either following
POINT OF SALES>CONFIGURATION>PAYMENT
METHOD Or
ACCOUNTING>CONFIGURATION>JOURNALS

Now once the Payment Method is created as described above, we need
to add the new Payment Method to the Point Of Sales by following,
POINT OF SALES>CONFIGURATION>POINT OF SALES. Here
we need add the new payment method
under “AVAILABLE
PAYMENT METHODS”